1. Field of the Invention
This invention relates to a spark-ignited internal combustion engine, and more particularly to a spark-ignited internal combustion engine in which the spark plug is supported so that the igniting tip of the spark plug is centrally positioned in the combustion chamber.
2. Description of the Prior Art
There has been known a spark-ignited engine in which a spark plug is centrally located in the combustion chamber in order to improve ignition of the air-fuel mixture in the combustion chamber and propagation of flame. However, in the case of engines provided with an intake port of a wide cross section or with a plurality of intake ports in order to increase the volumetric efficiency, it is difficult to locate the spark plug at the center of the combustion chamber due to limited room on the intake port side of the cylinder head. In Japanese Unexamined Patent Publication No. 57(1982)-102506, there is disclosed an internal combustion engine in which the spark plug is obliquely supported on the exhaust port side, where the remaining room is relatively large, and only the igniting tip is centrally located in the combustion chamber. This engine has a pair of intake ports 3 and a pair of exhaust ports 4, as shown in FIGS. 9 and 10, extending from opposite sides of the cylinder head 1 to a combustion chamber 2. A spark plug 5 is obliquely supported between the exhaust ports 4 (where room for supporting the spark plug can be obtained relatively easily) so that the igniting tip 5a is centrally located in the combustion chamber 2. With this arrangement, the spark plug can be supported to centrally locate the igniting tip in the combustion chamber in order to improve ignition of the air-fuel mixture and propagation of flame even in engines having a wide intake port or a plurality of intake ports.
However, this arrangement is disadvantageous in that since the spark plug is supported near the exhaust port through which hot exhaust gas is discharged, the male screw portion of the spark plug screwed into the wall portion of the combustion chamber is adversely affected by the heat transmitted thereto from the exhaust port. This thermal effect is especially significant in the case of a multicylinder engine in which thermal load is heavy and promotes deterioration of the spark plug.